The present invention relates to an image reading device including a reading optical system which is able to read an image with a variable magnification.
In an image reading device including a reading optical system which is able to read an image with a variable magnification, a sub-scanning speed is determined at a designated magnification ratio (set value). This magnification ratio may be made variable by means of a zoom lens or made variable by moving the conjugation length of a single-focus lens.
However, in a case where the zoom lens is used or in a case where the single-focus lens is used, there is a problem that, when a reading magnification ratio deviates from the set value due to a mechanical error and the sub-scanning speed is determined with the designated magnification ratio kept intact, a state in which the aspect ratio of an image is disordered occurs as a consequence of a change of a relative relation between a sub-scanning magnification ratio and a main scanning magnification ratio since a line cycle of a main scanning is constant.
Particularly when executing partial magnification of an image, in general, it is often the case where a magnification ratio considerably greater than the magnification ratio in the case of normal paper size change is set. In this case, an error of the aspect ratio of the image tends to become relatively conspicuous on the image.